Goal
The project goal is to improve oil recovery from fractured reservoirs through an improved fundamental understanding of the process of spontaneous imbibition by which oil is displaced from the rock matrix into fractures.

Performer
University of Wyoming, Laramie, WY

Background
Massive accumulations of oil are held in fractured, low-permeability reservoirs. Injection of brine results in rapid recovery of oil contained in the fractures. Additional and substantially greater oil recovery depends on oil displacement by the spontaneous imbibition of brine into the rock matrix and expulsion of oil via the fracture face. Spontaneous imbibition can be the determining mechanism for economic production. Spontaneous imbibition occurrence depends on the natural surface free energy of the reservoir. However, knowledge of the reservoir surface energy and its relationship to rate and extent of oil recovery by spontaneous imbibition is surprisingly limited. Many key questions as to the fundamental mechanism of oil recovery and potential means of improving oil recovery by spontaneous imbibition need to be addressed.

The fundamental mechanism of oil recovery by spontaneous imbibition is being investigated by an interdisciplinary team. Capillary pressure data and the corresponding change in surface free energy that drives spontaneous imbibition are being measured. New imbibition data, including novel pressure measurements, are being obtained. A network/numerical model is being developed with special emphasis on how imbibition is initiated and on how saturation profiles develop over time.

The basic concepts and ideas are being applied to interpretation of spontaneous imbibition in mixed-wet rocks prepared by adsorption from crude oil. Applications to improve recovery from fractured reservoirs will be tested in three ways: 1) reduced resistance to oil production at the fracture face by lowering interfacial tension, 2) increased rate of imbibition by alteration of rock matrix wettability, and 3) promotion of imbibition through manipulation of injection brine composition.

Benefits
Measurements of oil recovery by spontaneous imbibition on core samples are a routine and widely applied procedure for characterization of wettability and prediction of oil recovery from fractured reservoirs. The results of this project are directly aimed at improved physical understanding of the imbibition process, improved interpretation of routinely measured data, and development of new approaches to increasing oil recovery from fractured low-permeability oil-wet reservoirs. Very large reservoirs of this type—from which only a small fraction of in place oil has been recovered—occur in Alaska, Wyoming, and other Lower 48 States. There is strong industry interest in prediction and improvement of oil recovery from fractured reservoirs, which ultimately would boost U.S. oil reserves and production.

Results
In the first phase of the project, a mathematical network/numerical model will be developed and tested against experimental results of recovery versus time over broad variation of key factors such as rock properties, fluid phase viscosities, sample size, shape and boundary conditions. Two fundamentally important, but not previously considered, parameters of spontaneous imbibition, the capillary pressure acting to oppose production of oil at the outflow face and the pressure in the non-wetting phase at the no-flow boundary versus time, were measured and modeled. Simulation and network models were also be tested against special case solutions provided by analytic models.

In the second stage of the project, application of the fundamental concepts developed in the first stage of the project was demonstrated. The fundamental ideas, measurements, and analytic/numerical modeling have been applied to mixed-wet rocks. Imbibition measurements included novel sensitive pressure measurements designed to elucidate the basic mechanisms that determine induction time and drive the very slow rate of spontaneous imbibition commonly observed for mixed-wet rocks. In further demonstration of concepts, three approaches to improved oil recovery from fractured reservoirs were tested; use of surfactants to promote imbibition in oil wet rocks by wettability alteration: manipulation of injection brine composition: reduction of the capillary back pressure which opposes production of oil at the fracture face.

Current Status (October 2008)
This project is complete. A final Technical Report describing the modeling and experimental efforts as been submitted to DOE and can be accessed below under “Additional Information.”

Funding
This project was selected in response to DOE’s Oil Exploration and Production solicitation DE-PS26-02NT15375, PRIME (Public Resources Invested in Management and Extraction), May 31, 2002.

Publications
Fischer, H. And Morrow, N.R., Scaling of oil recovery by spontaneous imbibition for wide variation in aqueous phase viscosity with glycerol as the viscosifying agent, Proceedings of the 8th International Symposium on Reservoir Wettability, Houston, TX, May 17-18, 2004 (JPSE in press).

Seth, S., and Morrow, N.R., “Efficiency of Conversion of Work of Drainage to Surface energy for Sandstone and Carbonates,” Proceedings of SPE Annual Technical Conference, San Antonio, TX, September 2006, SPE 102490.